Wireless communication techniques based on multiple subcarriers, such as an orthogonal frequency-division multiplexing (OFDM) technique, are gaining worldwide popularity due to their broad applications. For example, an OFDM based communication system may be used in a plurality of networks including Worldwide Interoperability for Microwave Access (WiMax) networks, Wireless Fidelity (Wi-Fi) networks, Wireless Broadband (WiBro) networks, etc.
A transmitter in an OFDM based communication system may use a plurality of closely-spaced orthogonal subcarriers to carry data. For example, the transmitter may allocate the data on a plurality of parallel data channels, one for each of the subcarriers. Each of the subcarriers may be modulated with a conventional modulation scheme, e.g., quadrature amplitude modulation, at a relatively low symbol rate. In addition, the transmitter may perform an inverse fast Fourier transform (IFFT) on OFDM symbols representing the data to be transmitted, and transmit signals including the OFDM symbols to a receiver in the communication system. The receiver may perform a fast Fourier transform (FFT) on received signals to recover the OFDM symbols and, hence, the data.
The signals are transmitted from the transmitter to the receiver through a communication channel. In reality, the communication channel may have an effect on the signals when the signals are transmitted. The receiver may need knowledge of the communication channel to remove such effect, in order to accurately recover the data. To facilitate estimation of the communication channel, signals known to both the transmitter and the receiver, i.e., pilot symbols, may be inserted in OFDM symbols at the transmitter, such that the OFDM symbols include both data symbols and pilot symbols. The receiver may perform channel estimation based on resource units, also known as resource blocks, in the received signals, and each of the resource units includes a plurality of OFDM symbols and pilot symbols.
Typically, the transmitter is configured to transmit multiple data streams, such as first and second data streams. Accordingly, the transmitter is configured to allocate to ones of the subcarriers the respective pilot symbols for the first and second data streams, when the transmitter transmits the first and second data streams. However, in reality, the transmitter may transmit a reduced number of data streams, such as only the first data stream. In such a situation, the subcarriers that would otherwise carry the pilot symbols for the second data stream may become null subcarriers, i.e., carry no pilot or data symbol.
In addition, to increase a diversity of a channel coding block, e.g., a forward error correction (FEC) block, in the communication system, subcarrier permutation may be performed on data symbols. For example, the subcarrier permutation may be performed by changing locations of data symbols in, e.g., each resource unit.